The present section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present disclosure that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the following, a picture contains one or several arrays of samples (pixel values) in a specific picture/video format which specifies all information relative to the pixel values of a picture (or a video) and all information which may be used by a display and/or any other device to visualize and/or decode a picture (or video) for example. A picture comprises at least one component, in the shape of a first array of samples, usually a luma (or luminance) component, and, possibly, at least one other component, in the shape of at least one other array of samples, usually a color component. Or, equivalently, the same information may also be represented by a set of arrays of color samples, such as the traditional tri-chromatic RGB representation.
A pixel value is represented by a vector of n values, where n is the number of components. Each value of a vector is represented with a number of bits which defines a maximal dynamic range of the pixel values.
Standard-Dynamic-Range pictures (SDR pictures) are pictures whose luminance values are represented with a limited number of bits (most often 8 or 10). This limited representation does not allow correct rendering of small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range pictures (HDR pictures), the signal representation is extended in order to maintain a high accuracy of the signal over its entire range. In HDR pictures, pixel values are usually represented in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or in integers with a long representation, typically at least 16 bits.
A color gamut is a certain complete set of colors. The most common usage refers to a set of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device.
A color volume is defined by a color space and a dynamic range of the values represented in said color space.
For example, a color volume is defined by a RGB ITU-R Recommendation BT.2020 color space and the values represented in said RGB color space belong to a dynamic range from 0 to 4000 nits (candela per square meter). Another example of color volume is defined by a RGB BT.2020 color space and the values represented in said RGB color space belong to a dynamic range from 0 to 1000 nits.
Color-grading a picture (or a video) is a process of altering/enhancing the colors of the picture (or the video). Usually, color-grading a picture involves a change of the color volume (color space and/or dynamic range) or a change of the color gamut relative to this picture. Thus, two different color-graded versions of a same picture are versions of this picture whose values are represented in different color volumes (or color gamut) or versions of the picture whose at least one of their colors has been altered/enhanced according to different color grades. This may involve user interactions.
For example, in cinematographic production, a picture and a video are captured using tri-chromatic cameras into RGB color values composed of 3 components (Red, Green and Blue). The RGB color values depend on the tri-chromatic characteristics (color primaries) of the sensor. A first color-graded version of the captured picture is then obtained in order to get theatrical renders (using a specific theatrical grade). Typically, the values of the first color-graded version of the captured picture are represented according to a standardized YUV format such as BT.2020 which defines parameter values for Ultra-High Definition Television systems (UHDTV).
Then, a Colorist, usually in conjunction with a Director of Photography, performs a control on the color values of the first color-graded version of the captured picture by fine-tuning/tweaking some color values in order to instill an artistic intent.
A second color-graded version of the captured picture is also obtained to get home release renders (using specific home, Blu-Ray Disk/DVD grade). Typically, the values of the second color-graded version of the captured picture are represented according to a standardized YUV format such as ITU-R Recommendation BT.601 (Rec. 601) which defines studio encoding parameters of Standard Digital Television for standard 4:3 and wide-screen 16:9 aspect ratios, or ITU-R Recommendation BT.709 which defines parameter values for High Definition Television systems (HDTV).
Obtaining such a second color-graded version of the captured picture usually comprises stretching the color volume of the first color-graded version of the captured picture (for example RGB BT.2020 1000 nits modified by the Colorist) in order that the second color-graded version of the captured picture belong to a second color volume (RGB BT.709 1000 nits for example). This is an automatic step which uses a default color mapping function (for example for mapping of RGB BT.2020 format to RGB BT.709) usually approximated by a three dimensional look-up-table (also called 3D LUT). Note that all the considered YUV formats are characterized with the Color primaries parameters that allow defining any RGB-to-YUV and YUV-to-RGB color mappings.
Then, a Colorist, usually in conjunction with a Director of Photography, performs a control on the color values of the second color-graded version of the captured picture by fine-tuning/tweaking some color values in order to instill the artistic intent in the home release.
It is known to explicitly signal a default color mapping to a display, such as the YUV-to-RGB color mapping, so that the display is able to apply the appropriate default color mapping. Moreover, when the color mapping uses color mapping parameters calculated from a first and second color-graded version of a picture, it is known that those color mapping parameters are also signaled to the display so that the display is able to apply the appropriate default color mapping with appropriate color mapping parameters.
Using a default color mapping fails to preserve the artist intent because some colors, as specified by the colorist, in the first or second color-graded version of a picture may not be preserved when the default color mapping is applied on the first color-graded version of the picture.
For example, memory color such as flesh or skin tones, blue sky or green grass shades . . . etc, should be preserved when specified by the colorist for a given grade.
A typical use case is as follows: One has bought a new movie on a UHD HDR WCG Blu-ray disk ( ). Moreover, he is equipped in home with a UHD Blu-Ray player adapted to decode a HDR version burnt on said Blu-Ray disc. However, when the Blu-Ray player is connected to a legacy TV set that does not have HDR capability (but that may be 4K UHD and or WCG enabled as a DVB UHD-1 phase 1 compliant CE device). It means that such a Blu-Ray player has to convert the HDR version of the movie to a SDR version e.g. HDTV video signal for the legacy TV set. This conversion is said “blind” as the Blu-Ray player converts the HDR version of the movie without any knowledge of the “true” targeted movie look (e.g. current Blu-ray disc HD grade). Indeed, artistic intent (non-deterministic processing) may diverge according to the creative intent of the artist/colorist/Director of Photography and the available palette of picture alteration. Consequently, the resulting SDR version of the movie does not preserved the artist intent.
The present disclosure has been devised with the foregoing in mind.